The invention relates to electrical heating devices for the heating or vaporization of fluid. It is concerned with such devices having at least one porous body of inter-felted polycrystalline metal whiskers or metallized non-metallic whiskers or filaments, joined together metallically at their points of contact. The body is traversed by the medium to be heated and is itself heated by being in contact with a wall of good heat-conducting material. This may itself be an electrical heating element or be arranged to be heated by one.
Porous bodies which consist of interconnected polycrystalline metal whiskers or metallized non-metallic whiskers or filaments, such as Al.sub.2 O.sub.3 whiskers, carbon, quartz or rock fibers, can be produced with an extremely large inner surface area for the volume of the envelope of the body. They have been used successfully as electrical heating resistance elements for the vaporization or atomizing of liquid fuels. Such direct heating by electric current in many cases causes trouble owing to the good electrical conductivity of such a porous body, it has been proposed to heat the body indirectly by electricity. The porous body is then arranged to be in heat-conductive contact with another element heated by electric current. It has been found that this indirect electric heating does indeed have considerable advantages from the point of view of the source of energy available, but by comparison with a whiskers skeleton heated by the direct passage of electric current it takes rather a long time to heat the fluid to a desired temperature, particularly when a cylindrical porous body of rather large diameter is used. This is because the central zone of such a body is warmed very much less strongly than the external zone, which is in the immediate vicinity of the heated wall surrounding the cylinder. A body with alternately arranged heating plates and whisker plates can produce more uniform heating of the whisker portions, but they are traversed radially from inside to outside and only a short dwell time is possible for the fluid to be heated. Such plates cannot be made with an unduly large diameter for constructional reasons. Therefore, in order to heat a determined volume of liquid in a given time to a desired temperature or even to vaporize it, the whisker plates must be heated to relatively high temperatures which, when heating or vaporizing hydrocarbons, intensifies their tendency to crack and therefore to form porechoking residues.